In a TASI system, speech bursts on individual channels are detected and interpolated for transmission via fewer transmission facilities than there are channels. Typically there may be twice as many channels as there are facilities. Channel-to-facility assignment information, indicating which channels are assigned to which facilities, is also transmitted via the facilities to a far-end TASI system to enable the received interpolated speech bursts to be correctly supplied to the respective channels.
In order to accommodate the time required for a speech detector to detect the start of a speech burst on a channel, for a processor to assign a free facility to the channel in response to such detection, and for the channel-to-facility assignment to be transmitted, whilst still transmitting the entire speech burst without clipping of the start of the burst, it is known to delay the speech signals of the channel by a fixed period before supplying them to the facility. For example, the TASI system described in Cannon et al. U.S. Pat. No. 4,147,896 issued Apr. 3, 1979 includes 32 ms. fixed delay buffers in this respect. "Freeze-out" occurs in a TASI system in the event that a speech burst is detected on a channel but there is no facility free to which the channel can be assigned; consequently the speech burst is not transmitted. For relatively large numbers of channels and facilities, for example if there are 48 channels and 24 facilities, the probability of freeze-out of individual speech bursts is very small. It is desirable, however, to provide TASI systems capable of operating with relatively small numbers of channels and facilities. As the numbers of channels and facilities decrease, the probability of freeze-out of individual speech bursts increases, so that freeze-out becomes a significant problem in providing such TASI systems.
In order to mitigate the problem of freeze-out, the TASI system described in the Cannon et al. patent, as is more fully described in Clingenpeel U.S. Pat. No. 4,184,051 issued Jan. 15, 1980, includes variable delay buffers in addition to the fixed delay buffers already referred to. In the event that no facility is free for transmission of a speech burst on a channel, the speech signals of the channel are supplied from the relevant fixed delay buffer to one of the variable delay buffers, in which they are stored commmencing at a specific starting address and from which they are read out and transmitted when one of the facilities becomes available. For example, there are two or four such variable delay buffers. Whilst such an arrangement mitigates the problem of freeze-out to a certain extent, it suffers from the disadvantage that the number of variable delay buffers provided, and their capacity, are selected in dependence upon the numbers of channels and facilities connected to the TASI system, so that expansion of the system to accommodate more channels and facilities may necessitate modification of the buffer arrangement. Furthermore, the limited storage capacity of the variable delay buffers can lead to a loss of speech signals in the event that there is a relatively long delay before a facility becomes free. In addition, because the number of variable delay buffers is limited, the problem of freeze-out is not entirely avoided. Thus whenever a speech burst occurs on a channel when no facility is free and each of the variable delay buffers is already in use, then the speech burst is frozen out in the same manner as occurs when no variable delay buffers are present. This is the case even though each of the variable delay buffers may have a very large part of its storage capacity unused. Thus even though each variable delay buffer may have a large storage capacity, this capacity is relatively poorly utilized in attempting to avoid freeze-out.
An object of this invention, therefore, is to provide an improved method of buffering speech signals in a TASI system, and to provide an improved TASI system, which can be used with relatively small or large numbers of channels and facilities without modification of the system, and which provides for storage of speech signals in such a manner that the problems of clipping of speech signals and freeze-out of individual speech bursts can be substantially reduced.